(1) Field of the Invention
The present invention generally relates to failure detection devices for evaporative fuel purge systems, and more particularly to a failure detection device for an evaporative fuel purge system which device detects a failure of passage in an evaporative fuel purge system by detecting pressure of an evaporative fuel in the passage.
(2) Description of the Related Art
Generally, the fuel vapor evaporated in the fuel tank is adsorbed by the adsorbent in the canister so as to prevent the fuel from escaping into the atmosphere. However, the amount of fuel adsorbed in the canister is limited because the capacity of the canister is limited. Therefore, there is a purge system for fuel vapor which purges the fuel vapor adsorbed in the canister to an intake line of the engine in order to prevent overflow of fuel in the canister. The fuel vapor flows through the purge passage connecting the canister with the intake line of the engine and is purged to the inside of the intake line by a vacuum pressure generated by the engine operation. A purge control valve is usually provided to the purge passage to control the timing of the purge.
In this evaporative fuel purge system, there is a possibility that the fuel in the canister overflows or the fuel leaks to the atmosphere when the failure such as a fracture or a disconnection of the vapor line occurs. For this reason, an evaporative fuel purge system having a self diagnosis device for failures is suggested.
Conventionally, such an evaporative fuel purge system is disclosed in, for example, Japanese Laid-Open Patent Application No. 2-130255. The failure detection system disclosed in the Patent Publication above is for detecting a failure of the evaporative fuel purge system on the basis of a pressure change obtained, during purging, and before and after operation of the purge control valve, by providing a pressure sensor positioned between the canister and purge control valve on the vapor line.
However, in the aforementioned conventional device, there is a problem in that the failure can not be detected in case the failure occurs in the vapor passage which extending from the fuel tank to the canister, because the pressure sensor is provided in the purge passage.
Then detecting failure of the vapor passage as well as the purge passage, a pressure sensor is required in the vapor passage in addition to the pressure sensor in the purge passage; this results in an increase of the number of parts and in the complexity of the construction.
Moreover, there is a problem in that the construction of the failure detection means, which detects a failure on the basis of the signal from the pressure sensor, becomes complex.